This invention describes a new instrument by which potentially hazardous individual airborne fibres, such as those of asbestos, may be detected in real-time within an ambient environment. The instrument uses a rapid analysis of the spatial laser scattering profile (i.e: the complex manner in which individual particles scatter laser light) recorded from individual airborne particles, as a means of classifying the particles in terms of their morphological characteristics. The instrument incorporates a dedicated detector array chip to record the spatial scattering profiles from individual particles at high throughput rates and dedicated electronic processing routines to establish the possible presence of hazardous fibres.
The in situ detection of potentially hazardous respirable fibres has become a growing concern within industrialised countries as the health risks associated with these fibres have become more fully understood. The most commonly encountered hazardous fibres are of asbestos materials that, despite a wide-spread ban on their use for many years, are still present in vast quantities in public and commercial buildings and plants throughout the world. The most abundant asbestos mineral, Chrysotile (or white) asbestos, is present in over 95% of these installations. The second most commonly found variety is Crocidolite (or blue) asbestos, with Amosite (or brown) asbestos being a third but much rarer form. Crocidolite and Amosite belong to the amphibole class and are characterised by the fine, straight, needlelike fibres produced when the material is fragmented. Chrysotile asbestos belongs to the serpentine class of minerals and is characterised by a natural curvature in the fibres it produces. All three materials produce fibres that are capable of penetrating deep into the lung and that, because of their shape, become entrapped there. Crocidolite and Amosite fibres are known to be far more carcinogenic than those of Chrysotile asbestos, and although the exact reasons for this are still not confirmed, the half-life of the fibres in the lung (a function of the body""s ability to chemically dissolve the fibres) is believed to play a major role since this may be measured in decades for amphibole fibres compared with months for Chrysotile fibres.
Airborne asbestos fibre is a significant health hazard. Peto et al (Peto, J., Hodgson, J. T., Matthews, F. E. and Jones, J. R. The Lancet. 345, 535-539, Mar. 4, 1995), for example, highlight the continuing increase in mesothelioma mortality in Britain as a result of respirable asbestos fibres generated during clearance operations or routine building maintenance work The unambiguous confirmation of the presence of airborne asbestos fibres within an occupational environment can normally only be achieved by the use of filter cassette sampling of airborne particles followed by electron microscopy and, to determine chemical identity, a technique such as energy dispersive X-ray analysis. These processes are laborious and expensive to perform, and perhaps most importantly, provide results only many hours after the sample acquisition and possible personnel exposure has occurred. Several attempts have therefore been made to develop methods by which real-time or in-situ detection of airborne asbestos may be achieved. Rood et al [AP Rood, E J Walker and D Moore, xe2x80x9cConstruction of a portable fibre monitor measuring the differential light scattering from aligned fibresxe2x80x9d, in Proceedings of the International Symposium: Clean Air at Work, R H Brown, M Curtis, K J Saunders, and S Vandrendreissche, eds (Royal Society of Chemistry, London, 1992), pp 265-267] for example, have described a low cost portable fibre monitor developed at the UK Health and Safety Executive laboratories. This device is based on the differential light scattering produced by fibrous particles which are deposited electrostatically in uniform alignment onto a glass substrate. The device is capable of detecting fibrous particles but is not designed to detect individual particles, relying on the summation of scattering signals from a substantial number of deposited fibres in order to achieve a detectable signal. Rood states that the UK clearance limit for asbestos in buildings of 10 fibres per liter of air can be detected after about 300 minutes sampling time. This does not therefore constitute a real-time detection technique.
Another example is the comparatively widely used FAM-7400 Fibrous Aerosol Monitor (Mie Inc., Bedford, Mass.) developed originally by Lilienfeld et al. (Lilienfeld, P., Elterman, P., and Baron P. A. Ind. Hyg. Assoc. J. 40, 4, 270-282, 1979). This instrument draws air containing the airborne particles into a laser scattering chamber where the particles are carried along a horizontal glass tube coaxial with an illuminating laser beam. The particles remain in the beam for a comparatively long period, approximately 0.1 seconds, and many particles may be illuminated simultaneously. Around the glass tube is a quadruple electrode arrangement. By applying a time varying signal to the electrodes, the electric field within the tube causes electrically conducting fibres present in the air-flow to oscillate. The consequent cyclic variation in light scattered by the fibres to a single light detector at the side of the chamber is used to assess fibre concentration in the air. The FAM-7400 has several limitations (described in, for example xe2x80x98Aerosol Measurementxe2x80x99 by Willeke K. and Baron P. A., Van Nostrand Reinhold, 1993, pp 403-408): its sample volume flow rate through the laser beam is very low, resulting in comparatively long response times at low fibre concentrations (typically requiring 10 minutes to count 10 fibres at a concentration of 0.1 fibres/ml); it may classify as fibres non-fibrous particles which happen to oscillate in the applied electric field; since more than one fibre may be present in the beam at a given time, it can only estimate the number of fibres by the magnitude of the oscillation signal, and this requires some assumptions about the sizes of the fibres present; and it has reduced sensitivity for fibres which exhibit a natural curved morphology, such as the most common asbestos form, Chrysotile.
In theory, the detailed spatial intensity distribution of light scattered by individual particles (the scattering profile) contains information relating to inter alia the particle""s size, its shape, and its orientation with respect to the incident illumination. The invention reported here is aimed at exploiting this fact with a view to discriminating, in real-time, individual respirable hazardous fibres, such as asbestos, from other particles within an ambient environment.
Most optical scattering instruments used for particle counting and/or sizing, rely on collecting the scattered light with a single discrete detector. Such instruments cannot provide information on particle shape, and indeed normally assume that all measured particles are spherical when ascribing a size value to them. When a small number of discrete detectors are used, each collecting light over a different solid angle within the sphere of scattering around the particle, some shape as well as size information is obtainable. This principle is embodied in a number of patented instruments which may be considered as prior art: (xe2x80x98Portable Particle Analysersxe2x80x99. Ludlow, I. K. and Kaye P H. European Patent EP 0 316 172, July 1992; xe2x80x98Portable Particle Analysers Having Plural Detectorsxe2x80x99. Kaye P H and Ludlow I K U.S. Pat. No. 5,043,591 August 1991; xe2x80x98Particle Asymmetry Analyser having Sphericity Detectorsxe2x80x99. Kaye, P. H. and Ludlow, I. K. U.S. Pat. No. 5,089,714. February 1992; xe2x80x98Particle Asymmetry Analyserxe2x80x99. Ludlow, I. K. and Kaye, P. H. European Patent EP 0 316 171, September 1992; xe2x80x98Analysis of Particle Characteristicsxe2x80x99. Kaye, P. H., and Hirst, E. UK Patent GB 2278679B).
However, in order to extract more subtle information relating to particle morphology which may aid particle discrimination, the spatial intensity distribution of light scattered by the particle must be determined in more detail. If a particle is illuminated by a collimated light beam such as that from a laser, it will scatter light in all directions. FIG. 1 shows examples of forward scattering (i.e: below 35xc2x0 to the incident beam direction) recorded from various types of particle. These images were recorded using a laser scattering instrument fitted with a high-speed intensified charge-coupled-device (CCD) camera to record the light scatter data. In the instrument, the airborne particles are carried through an illuminating beam in single file by a laminar flow delivery system. The particles were illuminated by a 5-mW, 670 nm diode laser. This delivery system imposes aerodynamic forces upon the particles which cause fibrous or elongated particles to align preferentially with their long axis parallel to the flow, ie: orthogonally to the laser beam. The camera captures the distribution of light scattered by the particle throughout the angular range 5xc2x0 to 35xc2x0 to the illuminating beam direction.
The scattering profile examples given in FIG. 1 are recorded from typical background outdoor air (which contains a wide variation of particle types including droplets, irregular cubic particles, and occasional fibrous particles); from Crocidolite (or blue) asbestos; and from Chrysotile (or white) asbestos. Because elongated particles tend to align with the airflow (which for the examples shown was vertical), the fibres thus tend to traverse the laser beam vertically with the consequence that the scattering is predominantly in the horizontal plane, as illustrated in the Crocidolite and Chrysotile examples of FIG. 1. The data show in FIG. 1 illustrate the way in which scattering profiles, since they relate closely to the morphology or shape of the particles which produced them, may be used to discriminate between particle species, such as varieties of asbestos fibre, which exhibit very characteristic morphological features.
Typical scattering profiles from background particles, shown in the top row of FIG. 1, produce very variable profiles with few interpretable features since the particles which produced them are generally of irregular compact form. In contrast, the profiles produced by Crocidolite fibres, shown in the middle row, exhibit clearly discernible features: the profiles are generally of the form of a horizontal bar of scattering passing through the centre of the profile. The near horizontal form is as a result of the substantially vertical orientation of the fibre in the laser beam. The scattering is very localized as a result of the characteristic needle-like shape of the Crocidolite fibres, with virtually all the scattered light lying within a substantially horizontal bar. The total amount of scattered light may be related to a first order to the size (volume) of the scattering fibre, and the thickness of the scattering bar to the length-to-thickness aspect ratio of the fibre (higher aspect ratio fibres produce thinner scattering bars). It is therefore possible from each profile to estimate both the size and shape of the fibre which produced it, and this information is of great importance in the monitoring of hazardous respirable fibres such as asbestos since these parameters are known to significantly influence the degree of threat posed by the fibres upon inhalation.
Additionally, the bottom row of FIG. 1 illustrates the scattering from Chrysotile asbestos fibres which, being normally curved, cause the scattering profiles to assume a characteristic xe2x80x98bow-tiexe2x80x99 appearance. Here the scattering is still predominantly horizontal but the differing inclinations of incremental sections of fibre length to the incident illumination cause the fine divergent structure shown. The examples given in FIG. 1 illustrate the differences in the forms of the scattering profiles which exist for different particle morphologies, and indicate that this type of scattering profile offers the prospect of (i) discriminating asbestos-like fibres from background airborne particulates, (ii) the possible discrimination between serpentine (curved) and amphibole (straight) asbestos fibres, the latter being of higher carcinogenicity, and (iii) an estimate of the fibre size and shape and therefore potential threat posed by inhalation.
According to a first aspect of the present invention there is provided a fibre detector assembly comprising:
(i) a scattering chamber body;
(ii) means for drawing airborne particles through said body chamber, said means being adapted such the the particles tend to travel in single file with the longitudinal axis of particles with elongate shape substantially aligned with the direction of the air flow;
(iii) means for illuminating the particle stream within the chamber body;
(iv) an optical detector adapted to intercept and collect a portion of the light scattered by particles passing through the illuminating beam;
(v) data processing means adapted to capture and process the signals from the optical detector;
characterised in that the optical detector comprises a photodiode array consisting of a central opaque area surrounded by two or more annular rings of detector elements.
This arrangement provides for the real-time measurement of hazardous particles in a working environment. The special detector array makes this possible for the first time.
Preferably the detector array comprises three concentric annular rings of detector elements. The concentric array arrangement means facilitates the gathering of scattering data in an easily manageable form.
In a further preferred embodiment the first or innermost annular ring comprises a single detector and the second and subsequent annular rings each consist of a plurality of detector elements.
Preferably the radial interfaces between detector elements or segments in adjacent annular rings are out of phase. This then minimizes the possibility of fine fibre scattering from elongated fibres lying entirely along the xe2x80x98dead-zonesxe2x80x99 between adjacent detector elements in both the A and B segmented rings, and the commensurate possibility that fibre detection could be compromised.
In a particularly preferred embodiment the optical detector comprises three annular rings and the two outermost rings are divided into 16 segments or elements.
Preferably the annular rings of detector elements in the optical detector are substantially circular. A circular arrangement with radial segments is an efficient arrangement for detecting and gathering scattered light.
Preferably the data processing means incorporates a pattern classifier, which preferably comprises a neural network.
Preferably the neural network is a radial basis function neural network.
According to a second aspect of the invention there is provided an optical detector suitable for use in a fibre detector assembly of the type in question comprising a photodiode array consisting of a central opaque area surrounded by two or more annular rings of detector elements.
Preferably the first or innermost annular ring comprises a single detector and the second and subsequent annular rings each consist of a plurality of detector elements.
In a preferred embodiment the radial interfaces between detector elements or segments in adjacent annular rings are out of phase.
In a particularly preferred embodiment the optical detector comprises 3 annular rings and the two outermost rings are divided into 16 segments or elements.
Preferably the annular rings of detector elements are substantially circular.
For the avoidance of doubt this invention includes the optical detector as an entity in its own right for installation into existing fibre detectors. The invention also includes a complete fibre detector and a method of detecting hazardous fibres using said detectors.